As shown in Patent Document 1 (Japanese Utility Model Registration No. 3027420) for instance, a connection between a microphone unit and a microphone cord of a condenser microphone is normally made by electrically connecting two cores consisting of a signal wire and a power wire of a connecting cord to a predetermined position on an impedance converter side respectively and soldering a shielded wire on a predetermined ground side.
Tiepin-type and headset-type condenser microphones are required to be rather unnoticeable in use and lightweight, and so they are designed so that an outside diameter of the microphone unit becomes 5 mm or less for instance. In the case of such a small condenser microphone, the microphone unit contains a condenser portion and an impedance converter, and an inputted sound signal is connected to a power module separately placed via a dedicated connecting cable. The power module contains main circuits such as a low-cut circuit and an output circuit for driving the microphone therein. As for the connecting cable, a microphone cable consisting of a two-core shielded covered wire is used.
FIG. 2 is a relevant part enlarged view showing a conventional example of a coupling structure of this small microphone unit and the connecting cable. In this example, a connecting cable 2 uses a power wire 4 for supplying power to a microphone unit 1, a signal wire 5 for sending a sound signal outputted from the impedance converter not shown to the power module side and a shielded wire 6 for electrostatically shielding and grounding the power wire 4 and signal wire 5, which are covered by a sheath 3.
The connecting cable 2 has the power wire 4, signal wire 5 and shielded wire 6 projected on its one end 2a side electrically connected by soldering them to the predetermined positions on the microphone unit 1 side respectively. A joint between the microphone unit 1 and the connecting cable 2 is protected by covering and solidifying the portion between a back end side of the microphone unit 1 and the one end 2a side of the connecting cable 2 with an appropriate synthetic resin material 7 such as polypropylene to couple them integrally for protection.
In the case of the small condenser microphone 1 shown in FIG. 2, however, the sound signal in the connecting cable 2 is apt to be transmitted in an unbalanced state and so it is weak against noise from outside unlike a large condenser microphone. Therefore, there is a problem that if a strong electromagnetic wave is applied to the connecting cable 2 from a cellular phone for instance, the electromagnetic wave goes inside the microphone unit 1 and the power module so as to be detected by a semiconductor and generate the noise.
In the case of the coupling structure shown in FIG. 2 in particular, the power wire 4 and signal wire 5 are not covered by the shielded wire 6 but are connected to the microphone unit 1 side in an exposed state. Besides, the joint is covered by the synthetic resin material 7 having no shielding effect so that, if the electromagnetic wave goes into the power wire 4 and signal wire 5, there is a possibility that a high-frequency current may go into the shielded wire 6 and induce the noise.